Industrial lift trucks with a position measuring device for determining the position of the load lifting device relative to a reference point on the lift truck in question are known.
For instance, DE 195 08 346 C1 discloses an industrial lift truck with a position measuring device for determining the lift height of an adjustable-height load lifting device. The load lifting device is driven by a hydraulic cylinder supplied by a hydraulic pump, which is driven by an electric motor. Starting from an initial position of the load lifting device, the rotations of the hydraulic pump are counted up in one direction of rotation and down in the opposite direction of rotation, and are evaluated in light of the overall efficiency of the lift system to determine the current lift height.
Moreover, with regard to industrial lift trucks, there has already been proposed the concept of providing the lifting frame for a load lifting device with proximity switches at predetermined intervals, which switches respond to a marking that is movable with the load lifting device in order to determine the current lift height of the load lifting device.
Known from DE 32 11 486 A1 is a forklift vehicle with the features mentioned at the outset, wherein the position measuring device includes a rotating disk with radial slits on its edge that is arranged on the shaft of a pinion provided in the upper region of the moving part of the lift mast to deflect a lift chain for the load carrying fork. An optical sensor arrangement with a light-emitting diode and a phototransistor is provided in the vicinity of the edge of the disk to measure the rotational motion of the disk. As the disk rotates, the light path formed by the light-emitting diode and the phototransistor is alternately unblocked by the edge slits and blocked by the teeth between the edge slits, so that the phototransistor delivers a pulsed electrical signal whose pulse count at any point corresponds to the angle of rotation of the disk and that of the chain sprocket that is rotationally fixed to the disk. The current change in lift height of the load carrying fork is determined from the angle of rotation of the chain sprocket that is engaged with the lift chain. Since the rotation of the chain sprocket when the load carrying fork is raised or lowered is determined by the length of the section of chain that passes over the chain sprocket, changes in the chain length such as those which frequently occur during operation under load will lead to errors in determining lift height. A further disadvantage of this known solution is that the sensor components (light-emitting diode, phototransistor, rotating disk) must be arranged on the moving part of the lift mast, since the chain sprocket attached to the rotating disk must be arranged on the moving lift mast section for functional reasons. This not only produces a design constraint, but is also invariably subject to the problems that arise when electrical signals are transmitted by moving sensors.